/etc/pam.d/atd is in at 3.1.20-3.1ubuntu2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 | #
# The PAM configuration file for the at daemon
#
@include common-auth
@include common-account
session required pam_loginuid.so
@include common-session-noninteractive
session required pam_limits.so
auth required pam_env.so user_readenv=1
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