  We present an undergraduate lab that investigates weak localization in thin
silver films. The films prepared in our lab have thickness, $a$, between 60-200
\AA, a mesoscopic length scale. At low temperatures, the inelastic dephasing
length for electrons, $L_{\phi}$, exceeds the thickness of the film ($L_{\phi}
\gg a$), and the films are then quasi-2D in nature. In this situation, theory
predicts specific corrections to the Drude conductivity due to coherent
interference between conducting electrons' wavefunctions, a macroscopically
observable effect known as weak localization. This correction can be destroyed
with the application of a magnetic field, and the resulting magnetoresistance
curve provides information about electron transport in the film. This lab is
suitable for Junior or Senior level students in an advanced undergraduate lab
course.
