  We propose a definitive test of whether plates involved in Casimir
experiments should be modeled with ballistic or diffusive electrons--a
prominent controversy highlighted by a number of conflicting experiments. The
unambiguous test we propose is a measurement of the Casimir force between a
disordered quasi-2D metallic plate and a three-dimensional metallic system at
low temperatures, in which disorder-induced weak localization effects modify
the well-known Drude result in an experimentally tunable way. We calculate the
weak localization correction to the Casimir force as a function of magnetic
field and temperature and demonstrate that the quantum interference suppression
of the Casimir force is a strong, observable effect. The coexistence of weak
localization suppression in electronic transport and Casimir pressure would
lend credence to the Drude theory of the Casimir effect, while the lack of such
correlation would indicate a fundamental problem with the existing theory. We
also study mesoscopic disorder fluctuations in the Casimir effect and estimate
the width of the distribution of Casmir energies due to disorder fluctuations.
