What is the burden of defending the Attorney General's decision to grant scope employment certification to an employee of the Government?

Once a person is deemed to be an employee of the Government, then the Westfall Act mandates the Attorney General to defend any civil action brought against the employee, 28 U.S.C. 2679(c), and "[u]pon certification by the Attorney General . . . any civil action or proceeding commenced upon such claim in a United States district court shall be deemed an action against the United States, 2679(d)(1). A party challenging "the Attorney General's decision to grant scope of employment certification, 'bears the burden of presenting evidence and disproving [that] decision . . . by a preponderance of the evidence.'" Jackson v. Tate, 648 F.3d 729, 735 (9th Cir. 2011) (alterations in original) (quoting Kashin v. Kent, 457 F.3d 1033, 1036 (9th Cir. 2006)).