There are many, many, many cites I could pull for this, but might as well grab the a pertinent graph where the Supreme Court considers the question of whether a seizure of contraband specifically authorized by an act of Congress at the border nonetheless violates the Fourth Amendment's prohibition on "unreasonable" search and seizure. After extensive analysis of relevant case law and the history of the United States they conclude:
Border searches, then, from before the adoption of the Fourth Amendment, have been considered to be "reasonable" by the single fact that the person or item in question had entered into our country from outside. There has never been any additional requirement that the reasonableness of a border search depended on the existence of probable cause. This longstanding recognition that searches at our borders without probable cause and without a warrant are nonetheless "reasonable" has a history as old as the Fourth Amendment itself. We reaffirm it now.
Reasonable people can disagree with the Supreme Court on whether their application of the Constitution is as broad or narrow as it should be in any particular circumstance, but it is fairly clear that they did not write "The Constitution does not apply at the border!"
