his case. As with all searches subject to Fourth Amendment review, the constitutionality of a border search turns on whether it is reasonable. See Brigham City, Utah v. Swan, 547 U.S. 398, 403. 126 S.Ct. 1943, 164 L.Ed.2d 650 (2006) ("ITIhe ultimate touchstone of the Fourth Amendment is 'reasonableness.'