e border and its functional equivalents, officers on roving patrol may stop vehicles" only with reasonable suspicion they contain illegal aliens): Almeida—Sanchez v. United States, 413 U.S. 266.272-76, 93 S.Ct. 2535, 37 L.Ed.2d 596 (1973) (distinguishing searches of vehicles at the border from a search that occurred 25 miles