I've seen contracts that say both "at will" and "14 days notice required". Not sure if those are in conflict.
It doesn't really matter, though. Always give 2 weeks notice, as professional courtesy, but also take any of your fragile or expensive stuff home first. Depending on your employer, you may not even get a chance to revisit your desk, and instead may be escorted to the door and met with a box of your belongings.
(Most employers don't pull that shit, of course, and instead are very happy to have you tie up loose ends and commit any relevant documentation before you go.)