What is the burden of proving consent to search?

"The government has the burden of proving, by a preponderance of the evidence, that a consent to search was voluntary." United States v. Isiofia, 370 F.3d 226, 230-31 (2d Cir. 2004). "Voluntariness is a question of fact determined by a totality of all the circumstances." Id. at 231 (internal quotation marks omitted). "The ultimate question presented is whether the officer had a reasonable basis for believing there had been consent to the search." United States v. Garcia, 56 F.3d 418, 423 (2d Cir. 1995) (internal quotation marks and alteration omitted).