diff --git a/google/cloud/servicecontrol_v1/services/quota_controller/async_client.py b/google/cloud/servicecontrol_v1/services/quota_controller/async_client.py
index 49a268d..d91c49b 100644
--- a/google/cloud/servicecontrol_v1/services/quota_controller/async_client.py
+++ b/google/cloud/servicecontrol_v1/services/quota_controller/async_client.py
@@ -19,13 +19,15 @@
 from typing import Dict, Sequence, Tuple, Type, Union
 import pkg_resources
 
-import google.api_core.client_options as ClientOptions  # type: ignore
+from google.api_core.client_options import ClientOptions  # type: ignore
 from google.api_core import exceptions as core_exceptions  # type: ignore
 from google.api_core import gapic_v1  # type: ignore
 from google.api_core import retry as retries  # type: ignore
 from google.auth import credentials as ga_credentials  # type: ignore
 from google.oauth2 import service_account  # type: ignore
 
+OptionalRetry = Union[retries.Retry, object]
+
 from google.cloud.servicecontrol_v1.types import metric_value
 from google.cloud.servicecontrol_v1.types import quota_controller
 from .transports.base import QuotaControllerTransport, DEFAULT_CLIENT_INFO
@@ -165,9 +167,9 @@ def __init__(
 
     async def allocate_quota(
         self,
-        request: quota_controller.AllocateQuotaRequest = None,
+        request: Union[quota_controller.AllocateQuotaRequest, dict] = None,
         *,
-        retry: retries.Retry = gapic_v1.method.DEFAULT,
+        retry: OptionalRetry = gapic_v1.method.DEFAULT,
         timeout: float = None,
         metadata: Sequence[Tuple[str, str]] = (),
     ) -> quota_controller.AllocateQuotaResponse:
@@ -185,7 +187,7 @@ async def allocate_quota(
         functionality.
 
         Args:
-            request (:class:`google.cloud.servicecontrol_v1.types.AllocateQuotaRequest`):
+            request (Union[google.cloud.servicecontrol_v1.types.AllocateQuotaRequest, dict]):
                 The request object. Request message for the
                 AllocateQuota method.
             retry (google.api_core.retry.Retry): Designation of what errors, if any,
diff --git a/google/cloud/servicecontrol_v1/services/quota_controller/client.py b/google/cloud/servicecontrol_v1/services/quota_controller/client.py
index b2a4e64..cb4e64b 100644
--- a/google/cloud/servicecontrol_v1/services/quota_controller/client.py
+++ b/google/cloud/servicecontrol_v1/services/quota_controller/client.py
@@ -30,6 +30,8 @@
 from google.auth.exceptions import MutualTLSChannelError  # type: ignore
 from google.oauth2 import service_account  # type: ignore
 
+OptionalRetry = Union[retries.Retry, object]
+
 from google.cloud.servicecontrol_v1.types import metric_value
 from google.cloud.servicecontrol_v1.types import quota_controller
 from .transports.base import QuotaControllerTransport, DEFAULT_CLIENT_INFO
@@ -339,7 +341,7 @@ def allocate_quota(
         self,
         request: Union[quota_controller.AllocateQuotaRequest, dict] = None,
         *,
-        retry: retries.Retry = gapic_v1.method.DEFAULT,
+        retry: OptionalRetry = gapic_v1.method.DEFAULT,
         timeout: float = None,
         metadata: Sequence[Tuple[str, str]] = (),
     ) -> quota_controller.AllocateQuotaResponse:
diff --git a/google/cloud/servicecontrol_v1/services/quota_controller/transports/base.py b/google/cloud/servicecontrol_v1/services/quota_controller/transports/base.py
index 3257a28..a3e7ff7 100644
--- a/google/cloud/servicecontrol_v1/services/quota_controller/transports/base.py
+++ b/google/cloud/servicecontrol_v1/services/quota_controller/transports/base.py
@@ -15,7 +15,6 @@
 #
 import abc
 from typing import Awaitable, Callable, Dict, Optional, Sequence, Union
-import packaging.version
 import pkg_resources
 
 import google.auth  # type: ignore
@@ -37,15 +36,6 @@
 except pkg_resources.DistributionNotFound:
     DEFAULT_CLIENT_INFO = gapic_v1.client_info.ClientInfo()
 
-try:
-    # google.auth.__version__ was added in 1.26.0
-    _GOOGLE_AUTH_VERSION = google.auth.__version__
-except AttributeError:
-    try:  # try pkg_resources if it is available
-        _GOOGLE_AUTH_VERSION = pkg_resources.get_distribution("google-auth").version
-    except pkg_resources.DistributionNotFound:  # pragma: NO COVER
-        _GOOGLE_AUTH_VERSION = None
-
 
 class QuotaControllerTransport(abc.ABC):
     """Abstract transport class for QuotaController."""
@@ -98,7 +88,7 @@ def __init__(
             host += ":443"
         self._host = host
 
-        scopes_kwargs = self._get_scopes_kwargs(self._host, scopes)
+        scopes_kwargs = {"scopes": scopes, "default_scopes": self.AUTH_SCOPES}
 
         # Save the scopes.
         self._scopes = scopes
@@ -131,29 +121,6 @@ def __init__(
         # Save the credentials.
         self._credentials = credentials
 
-    # TODO(busunkim): This method is in the base transport
-    # to avoid duplicating code across the transport classes. These functions
-    # should be deleted once the minimum required versions of google-auth is increased.
-
-    # TODO: Remove this function once google-auth >= 1.25.0 is required
-    @classmethod
-    def _get_scopes_kwargs(
-        cls, host: str, scopes: Optional[Sequence[str]]
-    ) -> Dict[str, Optional[Sequence[str]]]:
-        """Returns scopes kwargs to pass to google-auth methods depending on the google-auth version"""
-
-        scopes_kwargs = {}
-
-        if _GOOGLE_AUTH_VERSION and (
-            packaging.version.parse(_GOOGLE_AUTH_VERSION)
-            >= packaging.version.parse("1.25.0")
-        ):
-            scopes_kwargs = {"scopes": scopes, "default_scopes": cls.AUTH_SCOPES}
-        else:
-            scopes_kwargs = {"scopes": scopes or cls.AUTH_SCOPES}
-
-        return scopes_kwargs
-
     def _prep_wrapped_messages(self, client_info):
         # Precompute the wrapped methods.
         self._wrapped_methods = {
diff --git a/google/cloud/servicecontrol_v1/services/quota_controller/transports/grpc_asyncio.py b/google/cloud/servicecontrol_v1/services/quota_controller/transports/grpc_asyncio.py
index a513a09..e25b311 100644
--- a/google/cloud/servicecontrol_v1/services/quota_controller/transports/grpc_asyncio.py
+++ b/google/cloud/servicecontrol_v1/services/quota_controller/transports/grpc_asyncio.py
@@ -20,7 +20,6 @@
 from google.api_core import grpc_helpers_async  # type: ignore
 from google.auth import credentials as ga_credentials  # type: ignore
 from google.auth.transport.grpc import SslCredentials  # type: ignore
-import packaging.version
 
 import grpc  # type: ignore
 from grpc.experimental import aio  # type: ignore
diff --git a/google/cloud/servicecontrol_v1/services/service_controller/async_client.py b/google/cloud/servicecontrol_v1/services/service_controller/async_client.py
index fbcc3ee..1f13c8a 100644
--- a/google/cloud/servicecontrol_v1/services/service_controller/async_client.py
+++ b/google/cloud/servicecontrol_v1/services/service_controller/async_client.py
@@ -19,13 +19,15 @@
 from typing import Dict, Sequence, Tuple, Type, Union
 import pkg_resources
 
-import google.api_core.client_options as ClientOptions  # type: ignore
+from google.api_core.client_options import ClientOptions  # type: ignore
 from google.api_core import exceptions as core_exceptions  # type: ignore
 from google.api_core import gapic_v1  # type: ignore
 from google.api_core import retry as retries  # type: ignore
 from google.auth import credentials as ga_credentials  # type: ignore
 from google.oauth2 import service_account  # type: ignore
 
+OptionalRetry = Union[retries.Retry, object]
+
 from google.cloud.servicecontrol_v1.types import check_error
 from google.cloud.servicecontrol_v1.types import service_controller
 from .transports.base import ServiceControllerTransport, DEFAULT_CLIENT_INFO
@@ -165,9 +167,9 @@ def __init__(
 
     async def check(
         self,
-        request: service_controller.CheckRequest = None,
+        request: Union[service_controller.CheckRequest, dict] = None,
         *,
-        retry: retries.Retry = gapic_v1.method.DEFAULT,
+        retry: OptionalRetry = gapic_v1.method.DEFAULT,
         timeout: float = None,
         metadata: Sequence[Tuple[str, str]] = (),
     ) -> service_controller.CheckResponse:
@@ -192,7 +194,7 @@ async def check(
         `Cloud IAM <https://cloud.google.com/iam>`__.
 
         Args:
-            request (:class:`google.cloud.servicecontrol_v1.types.CheckRequest`):
+            request (Union[google.cloud.servicecontrol_v1.types.CheckRequest, dict]):
                 The request object. Request message for the Check
                 method.
             retry (google.api_core.retry.Retry): Designation of what errors, if any,
@@ -235,9 +237,9 @@ async def check(
 
     async def report(
         self,
-        request: service_controller.ReportRequest = None,
+        request: Union[service_controller.ReportRequest, dict] = None,
         *,
-        retry: retries.Retry = gapic_v1.method.DEFAULT,
+        retry: OptionalRetry = gapic_v1.method.DEFAULT,
         timeout: float = None,
         metadata: Sequence[Tuple[str, str]] = (),
     ) -> service_controller.ReportResponse:
@@ -261,7 +263,7 @@ async def report(
         `Google Cloud IAM <https://cloud.google.com/iam>`__.
 
         Args:
-            request (:class:`google.cloud.servicecontrol_v1.types.ReportRequest`):
+            request (Union[google.cloud.servicecontrol_v1.types.ReportRequest, dict]):
                 The request object. Request message for the Report
                 method.
             retry (google.api_core.retry.Retry): Designation of what errors, if any,
diff --git a/google/cloud/servicecontrol_v1/services/service_controller/client.py b/google/cloud/servicecontrol_v1/services/service_controller/client.py
index 0ab18a6..3b5f129 100644
--- a/google/cloud/servicecontrol_v1/services/service_controller/client.py
+++ b/google/cloud/servicecontrol_v1/services/service_controller/client.py
@@ -30,6 +30,8 @@
 from google.auth.exceptions import MutualTLSChannelError  # type: ignore
 from google.oauth2 import service_account  # type: ignore
 
+OptionalRetry = Union[retries.Retry, object]
+
 from google.cloud.servicecontrol_v1.types import check_error
 from google.cloud.servicecontrol_v1.types import service_controller
 from .transports.base import ServiceControllerTransport, DEFAULT_CLIENT_INFO
@@ -341,7 +343,7 @@ def check(
         self,
         request: Union[service_controller.CheckRequest, dict] = None,
         *,
-        retry: retries.Retry = gapic_v1.method.DEFAULT,
+        retry: OptionalRetry = gapic_v1.method.DEFAULT,
         timeout: float = None,
         metadata: Sequence[Tuple[str, str]] = (),
     ) -> service_controller.CheckResponse:
@@ -403,7 +405,7 @@ def report(
         self,
         request: Union[service_controller.ReportRequest, dict] = None,
         *,
-        retry: retries.Retry = gapic_v1.method.DEFAULT,
+        retry: OptionalRetry = gapic_v1.method.DEFAULT,
         timeout: float = None,
         metadata: Sequence[Tuple[str, str]] = (),
     ) -> service_controller.ReportResponse:
diff --git a/google/cloud/servicecontrol_v1/services/service_controller/transports/base.py b/google/cloud/servicecontrol_v1/services/service_controller/transports/base.py
index c6559eb..07eb753 100644
--- a/google/cloud/servicecontrol_v1/services/service_controller/transports/base.py
+++ b/google/cloud/servicecontrol_v1/services/service_controller/transports/base.py
@@ -15,7 +15,6 @@
 #
 import abc
 from typing import Awaitable, Callable, Dict, Optional, Sequence, Union
-import packaging.version
 import pkg_resources
 
 import google.auth  # type: ignore
@@ -37,15 +36,6 @@
 except pkg_resources.DistributionNotFound:
     DEFAULT_CLIENT_INFO = gapic_v1.client_info.ClientInfo()
 
-try:
-    # google.auth.__version__ was added in 1.26.0
-    _GOOGLE_AUTH_VERSION = google.auth.__version__
-except AttributeError:
-    try:  # try pkg_resources if it is available
-        _GOOGLE_AUTH_VERSION = pkg_resources.get_distribution("google-auth").version
-    except pkg_resources.DistributionNotFound:  # pragma: NO COVER
-        _GOOGLE_AUTH_VERSION = None
-
 
 class ServiceControllerTransport(abc.ABC):
     """Abstract transport class for ServiceController."""
@@ -98,7 +88,7 @@ def __init__(
             host += ":443"
         self._host = host
 
-        scopes_kwargs = self._get_scopes_kwargs(self._host, scopes)
+        scopes_kwargs = {"scopes": scopes, "default_scopes": self.AUTH_SCOPES}
 
         # Save the scopes.
         self._scopes = scopes
@@ -131,29 +121,6 @@ def __init__(
         # Save the credentials.
         self._credentials = credentials
 
-    # TODO(busunkim): This method is in the base transport
-    # to avoid duplicating code across the transport classes. These functions
-    # should be deleted once the minimum required versions of google-auth is increased.
-
-    # TODO: Remove this function once google-auth >= 1.25.0 is required
-    @classmethod
-    def _get_scopes_kwargs(
-        cls, host: str, scopes: Optional[Sequence[str]]
-    ) -> Dict[str, Optional[Sequence[str]]]:
-        """Returns scopes kwargs to pass to google-auth methods depending on the google-auth version"""
-
-        scopes_kwargs = {}
-
-        if _GOOGLE_AUTH_VERSION and (
-            packaging.version.parse(_GOOGLE_AUTH_VERSION)
-            >= packaging.version.parse("1.25.0")
-        ):
-            scopes_kwargs = {"scopes": scopes, "default_scopes": cls.AUTH_SCOPES}
-        else:
-            scopes_kwargs = {"scopes": scopes or cls.AUTH_SCOPES}
-
-        return scopes_kwargs
-
     def _prep_wrapped_messages(self, client_info):
         # Precompute the wrapped methods.
         self._wrapped_methods = {
diff --git a/google/cloud/servicecontrol_v1/services/service_controller/transports/grpc_asyncio.py b/google/cloud/servicecontrol_v1/services/service_controller/transports/grpc_asyncio.py
index 5dbd054..46f953d 100644
--- a/google/cloud/servicecontrol_v1/services/service_controller/transports/grpc_asyncio.py
+++ b/google/cloud/servicecontrol_v1/services/service_controller/transports/grpc_asyncio.py
@@ -20,7 +20,6 @@
 from google.api_core import grpc_helpers_async  # type: ignore
 from google.auth import credentials as ga_credentials  # type: ignore
 from google.auth.transport.grpc import SslCredentials  # type: ignore
-import packaging.version
 
 import grpc  # type: ignore
 from grpc.experimental import aio  # type: ignore
diff --git a/google/cloud/servicecontrol_v1/types/distribution.py b/google/cloud/servicecontrol_v1/types/distribution.py
index b8ea164..22c57bd 100644
--- a/google/cloud/servicecontrol_v1/types/distribution.py
+++ b/google/cloud/servicecontrol_v1/types/distribution.py
@@ -34,6 +34,13 @@ class Distribution(proto.Message):
        variance
     -  a histogram of the values of the sample points
 
+    This message has `oneof`_ fields (mutually exclusive fields).
+    For each oneof, at most one member field can be set at the same time.
+    Setting any member of the oneof automatically clears all other
+    members.
+
+    .. _oneof: https://proto-plus-python.readthedocs.io/en/stable/fields.html#oneofs-mutually-exclusive-fields
+
     Attributes:
         count (int):
             The total number of samples in the
@@ -68,10 +75,13 @@ class Distribution(proto.Message):
             Any suffix of trailing zeros may be omitted.
         linear_buckets (google.cloud.servicecontrol_v1.types.Distribution.LinearBuckets):
             Buckets with constant width.
+            This field is a member of `oneof`_ ``bucket_option``.
         exponential_buckets (google.cloud.servicecontrol_v1.types.Distribution.ExponentialBuckets):
             Buckets with exponentially growing width.
+            This field is a member of `oneof`_ ``bucket_option``.
         explicit_buckets (google.cloud.servicecontrol_v1.types.Distribution.ExplicitBuckets):
             Buckets with arbitrary user-provided width.
+            This field is a member of `oneof`_ ``bucket_option``.
         exemplars (Sequence[google.api.distribution_pb2.Exemplar]):
             Example points. Must be in increasing order of ``value``
             field.
diff --git a/google/cloud/servicecontrol_v1/types/log_entry.py b/google/cloud/servicecontrol_v1/types/log_entry.py
index 0ac9e95..dc8ec7f 100644
--- a/google/cloud/servicecontrol_v1/types/log_entry.py
+++ b/google/cloud/servicecontrol_v1/types/log_entry.py
@@ -31,6 +31,13 @@
 class LogEntry(proto.Message):
     r"""An individual log entry.
 
+    This message has `oneof`_ fields (mutually exclusive fields).
+    For each oneof, at most one member field can be set at the same time.
+    Setting any member of the oneof automatically clears all other
+    members.
+
+    .. _oneof: https://proto-plus-python.readthedocs.io/en/stable/fields.html#oneofs-mutually-exclusive-fields
+
     Attributes:
         name (str):
             Required. The log to which this log entry belongs. Examples:
@@ -63,12 +70,15 @@ class LogEntry(proto.Message):
             The log entry payload, represented as a protocol buffer that
             is expressed as a JSON object. The only accepted type
             currently is [AuditLog][google.cloud.audit.AuditLog].
+            This field is a member of `oneof`_ ``payload``.
         text_payload (str):
             The log entry payload, represented as a
             Unicode string (UTF-8).
+            This field is a member of `oneof`_ ``payload``.
         struct_payload (google.protobuf.struct_pb2.Struct):
             The log entry payload, represented as a
             structure that is expressed as a JSON object.
+            This field is a member of `oneof`_ ``payload``.
         operation (google.cloud.servicecontrol_v1.types.LogEntryOperation):
             Optional. Information about an operation
             associated with the log entry, if applicable.
diff --git a/google/cloud/servicecontrol_v1/types/metric_value.py b/google/cloud/servicecontrol_v1/types/metric_value.py
index 9f1e628..ed11979 100644
--- a/google/cloud/servicecontrol_v1/types/metric_value.py
+++ b/google/cloud/servicecontrol_v1/types/metric_value.py
@@ -27,6 +27,13 @@
 class MetricValue(proto.Message):
     r"""Represents a single metric value.
 
+    This message has `oneof`_ fields (mutually exclusive fields).
+    For each oneof, at most one member field can be set at the same time.
+    Setting any member of the oneof automatically clears all other
+    members.
+
+    .. _oneof: https://proto-plus-python.readthedocs.io/en/stable/fields.html#oneofs-mutually-exclusive-fields
+
     Attributes:
         labels (Sequence[google.cloud.servicecontrol_v1.types.MetricValue.LabelsEntry]):
             The labels describing the metric value. See comments on
@@ -48,14 +55,19 @@ class MetricValue(proto.Message):
             will be used.
         bool_value (bool):
             A boolean value.
+            This field is a member of `oneof`_ ``value``.
         int64_value (int):
             A signed 64-bit integer value.
+            This field is a member of `oneof`_ ``value``.
         double_value (float):
             A double precision floating point value.
+            This field is a member of `oneof`_ ``value``.
         string_value (str):
             A text string value.
+            This field is a member of `oneof`_ ``value``.
         distribution_value (google.cloud.servicecontrol_v1.types.Distribution):
             A distribution value.
+            This field is a member of `oneof`_ ``value``.
     """
 
     labels = proto.MapField(proto.STRING, proto.STRING, number=1,)
diff --git a/setup.py b/setup.py
index 074a894..f6d1288 100644
--- a/setup.py
+++ b/setup.py
@@ -28,10 +28,9 @@
     # NOTE: Maintainers, please do not require google-api-core>=2.x.x
     # Until this issue is closed
     # https://github.com/googleapis/google-cloud-python/issues/10566
-    "google-api-core[grpc] >= 1.26.0, <3.0.0dev",
+    "google-api-core[grpc] >= 1.28.0, <3.0.0dev",
     "libcst >= 0.2.5",
     "proto-plus >= 1.15.0",
-    "packaging >= 14.3",
 ]
 
 package_root = os.path.abspath(os.path.dirname(__file__))
diff --git a/testing/constraints-3.6.txt b/testing/constraints-3.6.txt
index 316c3c5..d5ee700 100644
--- a/testing/constraints-3.6.txt
+++ b/testing/constraints-3.6.txt
@@ -5,8 +5,6 @@
 #
 # e.g., if setup.py has "foo >= 1.14.0, < 2.0.0dev",
 # Then this file should have foo==1.14.0
-google-api-core==1.26.0
+google-api-core==1.28.0
 libcst==0.2.5
 proto-plus==1.15.0
-packaging==14.3
-google-auth==1.24.0  # TODO: remove when google-auth>=1.25.0 is transitively required through google-auth
diff --git a/tests/unit/gapic/servicecontrol_v1/test_quota_controller.py b/tests/unit/gapic/servicecontrol_v1/test_quota_controller.py
index 9f4ae58..3022024 100644
--- a/tests/unit/gapic/servicecontrol_v1/test_quota_controller.py
+++ b/tests/unit/gapic/servicecontrol_v1/test_quota_controller.py
@@ -15,7 +15,6 @@
 #
 import os
 import mock
-import packaging.version
 
 import grpc
 from grpc.experimental import aio
@@ -40,9 +39,6 @@
     QuotaControllerClient,
 )
 from google.cloud.servicecontrol_v1.services.quota_controller import transports
-from google.cloud.servicecontrol_v1.services.quota_controller.transports.base import (
-    _GOOGLE_AUTH_VERSION,
-)
 from google.cloud.servicecontrol_v1.types import distribution as gas_distribution
 from google.cloud.servicecontrol_v1.types import metric_value
 from google.cloud.servicecontrol_v1.types import quota_controller
@@ -52,20 +48,6 @@
 import google.auth
 
 
-# TODO(busunkim): Once google-auth >= 1.25.0 is required transitively
-# through google-api-core:
-# - Delete the auth "less than" test cases
-# - Delete these pytest markers (Make the "greater than or equal to" tests the default).
-requires_google_auth_lt_1_25_0 = pytest.mark.skipif(
-    packaging.version.parse(_GOOGLE_AUTH_VERSION) >= packaging.version.parse("1.25.0"),
-    reason="This test requires google-auth < 1.25.0",
-)
-requires_google_auth_gte_1_25_0 = pytest.mark.skipif(
-    packaging.version.parse(_GOOGLE_AUTH_VERSION) < packaging.version.parse("1.25.0"),
-    reason="This test requires google-auth >= 1.25.0",
-)
-
-
 def client_cert_source_callback():
     return b"cert bytes", b"key bytes"
 
@@ -222,7 +204,7 @@ def test_quota_controller_client_client_options(
     options = client_options.ClientOptions(api_endpoint="squid.clam.whelk")
     with mock.patch.object(transport_class, "__init__") as patched:
         patched.return_value = None
-        client = client_class(client_options=options)
+        client = client_class(transport=transport_name, client_options=options)
         patched.assert_called_once_with(
             credentials=None,
             credentials_file=None,
@@ -239,7 +221,7 @@ def test_quota_controller_client_client_options(
     with mock.patch.dict(os.environ, {"GOOGLE_API_USE_MTLS_ENDPOINT": "never"}):
         with mock.patch.object(transport_class, "__init__") as patched:
             patched.return_value = None
-            client = client_class()
+            client = client_class(transport=transport_name)
             patched.assert_called_once_with(
                 credentials=None,
                 credentials_file=None,
@@ -256,7 +238,7 @@ def test_quota_controller_client_client_options(
     with mock.patch.dict(os.environ, {"GOOGLE_API_USE_MTLS_ENDPOINT": "always"}):
         with mock.patch.object(transport_class, "__init__") as patched:
             patched.return_value = None
-            client = client_class()
+            client = client_class(transport=transport_name)
             patched.assert_called_once_with(
                 credentials=None,
                 credentials_file=None,
@@ -285,7 +267,7 @@ def test_quota_controller_client_client_options(
     options = client_options.ClientOptions(quota_project_id="octopus")
     with mock.patch.object(transport_class, "__init__") as patched:
         patched.return_value = None
-        client = client_class(client_options=options)
+        client = client_class(transport=transport_name, client_options=options)
         patched.assert_called_once_with(
             credentials=None,
             credentials_file=None,
@@ -354,7 +336,7 @@ def test_quota_controller_client_mtls_env_auto(
         )
         with mock.patch.object(transport_class, "__init__") as patched:
             patched.return_value = None
-            client = client_class(client_options=options)
+            client = client_class(transport=transport_name, client_options=options)
 
             if use_client_cert_env == "false":
                 expected_client_cert_source = None
@@ -396,7 +378,7 @@ def test_quota_controller_client_mtls_env_auto(
                         expected_client_cert_source = client_cert_source_callback
 
                     patched.return_value = None
-                    client = client_class()
+                    client = client_class(transport=transport_name)
                     patched.assert_called_once_with(
                         credentials=None,
                         credentials_file=None,
@@ -418,7 +400,7 @@ def test_quota_controller_client_mtls_env_auto(
                 return_value=False,
             ):
                 patched.return_value = None
-                client = client_class()
+                client = client_class(transport=transport_name)
                 patched.assert_called_once_with(
                     credentials=None,
                     credentials_file=None,
@@ -449,7 +431,7 @@ def test_quota_controller_client_client_options_scopes(
     options = client_options.ClientOptions(scopes=["1", "2"],)
     with mock.patch.object(transport_class, "__init__") as patched:
         patched.return_value = None
-        client = client_class(client_options=options)
+        client = client_class(transport=transport_name, client_options=options)
         patched.assert_called_once_with(
             credentials=None,
             credentials_file=None,
@@ -480,7 +462,7 @@ def test_quota_controller_client_client_options_credentials_file(
     options = client_options.ClientOptions(credentials_file="credentials.json")
     with mock.patch.object(transport_class, "__init__") as patched:
         patched.return_value = None
-        client = client_class(client_options=options)
+        client = client_class(transport=transport_name, client_options=options)
         patched.assert_called_once_with(
             credentials=None,
             credentials_file="credentials.json",
@@ -707,7 +689,6 @@ def test_quota_controller_base_transport():
         transport.close()
 
 
-@requires_google_auth_gte_1_25_0
 def test_quota_controller_base_transport_with_credentials_file():
     # Instantiate the base transport with a credentials file
     with mock.patch.object(
@@ -731,29 +712,6 @@ def test_quota_controller_base_transport_with_credentials_file():
         )
 
 
-@requires_google_auth_lt_1_25_0
-def test_quota_controller_base_transport_with_credentials_file_old_google_auth():
-    # Instantiate the base transport with a credentials file
-    with mock.patch.object(
-        google.auth, "load_credentials_from_file", autospec=True
-    ) as load_creds, mock.patch(
-        "google.cloud.servicecontrol_v1.services.quota_controller.transports.QuotaControllerTransport._prep_wrapped_messages"
-    ) as Transport:
-        Transport.return_value = None
-        load_creds.return_value = (ga_credentials.AnonymousCredentials(), None)
-        transport = transports.QuotaControllerTransport(
-            credentials_file="credentials.json", quota_project_id="octopus",
-        )
-        load_creds.assert_called_once_with(
-            "credentials.json",
-            scopes=(
-                "https://www.googleapis.com/auth/cloud-platform",
-                "https://www.googleapis.com/auth/servicecontrol",
-            ),
-            quota_project_id="octopus",
-        )
-
-
 def test_quota_controller_base_transport_with_adc():
     # Test the default credentials are used if credentials and credentials_file are None.
     with mock.patch.object(google.auth, "default", autospec=True) as adc, mock.patch(
@@ -765,7 +723,6 @@ def test_quota_controller_base_transport_with_adc():
         adc.assert_called_once()
 
 
-@requires_google_auth_gte_1_25_0
 def test_quota_controller_auth_adc():
     # If no credentials are provided, we should use ADC credentials.
     with mock.patch.object(google.auth, "default", autospec=True) as adc:
@@ -781,21 +738,6 @@ def test_quota_controller_auth_adc():
         )
 
 
-@requires_google_auth_lt_1_25_0
-def test_quota_controller_auth_adc_old_google_auth():
-    # If no credentials are provided, we should use ADC credentials.
-    with mock.patch.object(google.auth, "default", autospec=True) as adc:
-        adc.return_value = (ga_credentials.AnonymousCredentials(), None)
-        QuotaControllerClient()
-        adc.assert_called_once_with(
-            scopes=(
-                "https://www.googleapis.com/auth/cloud-platform",
-                "https://www.googleapis.com/auth/servicecontrol",
-            ),
-            quota_project_id=None,
-        )
-
-
 @pytest.mark.parametrize(
     "transport_class",
     [
@@ -803,7 +745,6 @@ def test_quota_controller_auth_adc_old_google_auth():
         transports.QuotaControllerGrpcAsyncIOTransport,
     ],
 )
-@requires_google_auth_gte_1_25_0
 def test_quota_controller_transport_auth_adc(transport_class):
     # If credentials and host are not provided, the transport class should use
     # ADC credentials.
@@ -820,29 +761,6 @@ def test_quota_controller_transport_auth_adc(transport_class):
         )
 
 
-@pytest.mark.parametrize(
-    "transport_class",
-    [
-        transports.QuotaControllerGrpcTransport,
-        transports.QuotaControllerGrpcAsyncIOTransport,
-    ],
-)
-@requires_google_auth_lt_1_25_0
-def test_quota_controller_transport_auth_adc_old_google_auth(transport_class):
-    # If credentials and host are not provided, the transport class should use
-    # ADC credentials.
-    with mock.patch.object(google.auth, "default", autospec=True) as adc:
-        adc.return_value = (ga_credentials.AnonymousCredentials(), None)
-        transport_class(quota_project_id="octopus")
-        adc.assert_called_once_with(
-            scopes=(
-                "https://www.googleapis.com/auth/cloud-platform",
-                "https://www.googleapis.com/auth/servicecontrol",
-            ),
-            quota_project_id="octopus",
-        )
-
-
 @pytest.mark.parametrize(
     "transport_class,grpc_helpers",
     [
diff --git a/tests/unit/gapic/servicecontrol_v1/test_service_controller.py b/tests/unit/gapic/servicecontrol_v1/test_service_controller.py
index 3e2934d..4b068e8 100644
--- a/tests/unit/gapic/servicecontrol_v1/test_service_controller.py
+++ b/tests/unit/gapic/servicecontrol_v1/test_service_controller.py
@@ -15,7 +15,6 @@
 #
 import os
 import mock
-import packaging.version
 
 import grpc
 from grpc.experimental import aio
@@ -40,9 +39,6 @@
     ServiceControllerClient,
 )
 from google.cloud.servicecontrol_v1.services.service_controller import transports
-from google.cloud.servicecontrol_v1.services.service_controller.transports.base import (
-    _GOOGLE_AUTH_VERSION,
-)
 from google.cloud.servicecontrol_v1.types import check_error
 from google.cloud.servicecontrol_v1.types import distribution as gas_distribution
 from google.cloud.servicecontrol_v1.types import http_request
@@ -59,20 +55,6 @@
 import google.auth
 
 
-# TODO(busunkim): Once google-auth >= 1.25.0 is required transitively
-# through google-api-core:
-# - Delete the auth "less than" test cases
-# - Delete these pytest markers (Make the "greater than or equal to" tests the default).
-requires_google_auth_lt_1_25_0 = pytest.mark.skipif(
-    packaging.version.parse(_GOOGLE_AUTH_VERSION) >= packaging.version.parse("1.25.0"),
-    reason="This test requires google-auth < 1.25.0",
-)
-requires_google_auth_gte_1_25_0 = pytest.mark.skipif(
-    packaging.version.parse(_GOOGLE_AUTH_VERSION) < packaging.version.parse("1.25.0"),
-    reason="This test requires google-auth >= 1.25.0",
-)
-
-
 def client_cert_source_callback():
     return b"cert bytes", b"key bytes"
 
@@ -230,7 +212,7 @@ def test_service_controller_client_client_options(
     options = client_options.ClientOptions(api_endpoint="squid.clam.whelk")
     with mock.patch.object(transport_class, "__init__") as patched:
         patched.return_value = None
-        client = client_class(client_options=options)
+        client = client_class(transport=transport_name, client_options=options)
         patched.assert_called_once_with(
             credentials=None,
             credentials_file=None,
@@ -247,7 +229,7 @@ def test_service_controller_client_client_options(
     with mock.patch.dict(os.environ, {"GOOGLE_API_USE_MTLS_ENDPOINT": "never"}):
         with mock.patch.object(transport_class, "__init__") as patched:
             patched.return_value = None
-            client = client_class()
+            client = client_class(transport=transport_name)
             patched.assert_called_once_with(
                 credentials=None,
                 credentials_file=None,
@@ -264,7 +246,7 @@ def test_service_controller_client_client_options(
     with mock.patch.dict(os.environ, {"GOOGLE_API_USE_MTLS_ENDPOINT": "always"}):
         with mock.patch.object(transport_class, "__init__") as patched:
             patched.return_value = None
-            client = client_class()
+            client = client_class(transport=transport_name)
             patched.assert_called_once_with(
                 credentials=None,
                 credentials_file=None,
@@ -293,7 +275,7 @@ def test_service_controller_client_client_options(
     options = client_options.ClientOptions(quota_project_id="octopus")
     with mock.patch.object(transport_class, "__init__") as patched:
         patched.return_value = None
-        client = client_class(client_options=options)
+        client = client_class(transport=transport_name, client_options=options)
         patched.assert_called_once_with(
             credentials=None,
             credentials_file=None,
@@ -362,7 +344,7 @@ def test_service_controller_client_mtls_env_auto(
         )
         with mock.patch.object(transport_class, "__init__") as patched:
             patched.return_value = None
-            client = client_class(client_options=options)
+            client = client_class(transport=transport_name, client_options=options)
 
             if use_client_cert_env == "false":
                 expected_client_cert_source = None
@@ -404,7 +386,7 @@ def test_service_controller_client_mtls_env_auto(
                         expected_client_cert_source = client_cert_source_callback
 
                     patched.return_value = None
-                    client = client_class()
+                    client = client_class(transport=transport_name)
                     patched.assert_called_once_with(
                         credentials=None,
                         credentials_file=None,
@@ -426,7 +408,7 @@ def test_service_controller_client_mtls_env_auto(
                 return_value=False,
             ):
                 patched.return_value = None
-                client = client_class()
+                client = client_class(transport=transport_name)
                 patched.assert_called_once_with(
                     credentials=None,
                     credentials_file=None,
@@ -457,7 +439,7 @@ def test_service_controller_client_client_options_scopes(
     options = client_options.ClientOptions(scopes=["1", "2"],)
     with mock.patch.object(transport_class, "__init__") as patched:
         patched.return_value = None
-        client = client_class(client_options=options)
+        client = client_class(transport=transport_name, client_options=options)
         patched.assert_called_once_with(
             credentials=None,
             credentials_file=None,
@@ -488,7 +470,7 @@ def test_service_controller_client_client_options_credentials_file(
     options = client_options.ClientOptions(credentials_file="credentials.json")
     with mock.patch.object(transport_class, "__init__") as patched:
         patched.return_value = None
-        client = client_class(client_options=options)
+        client = client_class(transport=transport_name, client_options=options)
         patched.assert_called_once_with(
             credentials=None,
             credentials_file="credentials.json",
@@ -807,7 +789,6 @@ def test_service_controller_base_transport():
         transport.close()
 
 
-@requires_google_auth_gte_1_25_0
 def test_service_controller_base_transport_with_credentials_file():
     # Instantiate the base transport with a credentials file
     with mock.patch.object(
@@ -831,29 +812,6 @@ def test_service_controller_base_transport_with_credentials_file():
         )
 
 
-@requires_google_auth_lt_1_25_0
-def test_service_controller_base_transport_with_credentials_file_old_google_auth():
-    # Instantiate the base transport with a credentials file
-    with mock.patch.object(
-        google.auth, "load_credentials_from_file", autospec=True
-    ) as load_creds, mock.patch(
-        "google.cloud.servicecontrol_v1.services.service_controller.transports.ServiceControllerTransport._prep_wrapped_messages"
-    ) as Transport:
-        Transport.return_value = None
-        load_creds.return_value = (ga_credentials.AnonymousCredentials(), None)
-        transport = transports.ServiceControllerTransport(
-            credentials_file="credentials.json", quota_project_id="octopus",
-        )
-        load_creds.assert_called_once_with(
-            "credentials.json",
-            scopes=(
-                "https://www.googleapis.com/auth/cloud-platform",
-                "https://www.googleapis.com/auth/servicecontrol",
-            ),
-            quota_project_id="octopus",
-        )
-
-
 def test_service_controller_base_transport_with_adc():
     # Test the default credentials are used if credentials and credentials_file are None.
     with mock.patch.object(google.auth, "default", autospec=True) as adc, mock.patch(
@@ -865,7 +823,6 @@ def test_service_controller_base_transport_with_adc():
         adc.assert_called_once()
 
 
-@requires_google_auth_gte_1_25_0
 def test_service_controller_auth_adc():
     # If no credentials are provided, we should use ADC credentials.
     with mock.patch.object(google.auth, "default", autospec=True) as adc:
@@ -881,21 +838,6 @@ def test_service_controller_auth_adc():
         )
 
 
-@requires_google_auth_lt_1_25_0
-def test_service_controller_auth_adc_old_google_auth():
-    # If no credentials are provided, we should use ADC credentials.
-    with mock.patch.object(google.auth, "default", autospec=True) as adc:
-        adc.return_value = (ga_credentials.AnonymousCredentials(), None)
-        ServiceControllerClient()
-        adc.assert_called_once_with(
-            scopes=(
-                "https://www.googleapis.com/auth/cloud-platform",
-                "https://www.googleapis.com/auth/servicecontrol",
-            ),
-            quota_project_id=None,
-        )
-
-
 @pytest.mark.parametrize(
     "transport_class",
     [
@@ -903,7 +845,6 @@ def test_service_controller_auth_adc_old_google_auth():
         transports.ServiceControllerGrpcAsyncIOTransport,
     ],
 )
-@requires_google_auth_gte_1_25_0
 def test_service_controller_transport_auth_adc(transport_class):
     # If credentials and host are not provided, the transport class should use
     # ADC credentials.
@@ -920,29 +861,6 @@ def test_service_controller_transport_auth_adc(transport_class):
         )
 
 
-@pytest.mark.parametrize(
-    "transport_class",
-    [
-        transports.ServiceControllerGrpcTransport,
-        transports.ServiceControllerGrpcAsyncIOTransport,
-    ],
-)
-@requires_google_auth_lt_1_25_0
-def test_service_controller_transport_auth_adc_old_google_auth(transport_class):
-    # If credentials and host are not provided, the transport class should use
-    # ADC credentials.
-    with mock.patch.object(google.auth, "default", autospec=True) as adc:
-        adc.return_value = (ga_credentials.AnonymousCredentials(), None)
-        transport_class(quota_project_id="octopus")
-        adc.assert_called_once_with(
-            scopes=(
-                "https://www.googleapis.com/auth/cloud-platform",
-                "https://www.googleapis.com/auth/servicecontrol",
-            ),
-            quota_project_id="octopus",
-        )
-
-
 @pytest.mark.parametrize(
     "transport_class,grpc_helpers",
     [