AHARONOV-BOHM EFFECT IN THE PRESENCE OF EVANESCENT MODES

It is known that differential magnetoconductance of a normal metal loo p connected to reservoirs by ideal wires is always negative when an el ectron travels as an evanescent modes in the loop. This is in contrast to the fact that the magnetoconductance for propagating modes is very sensitive to small changes in geometric details and the Fermi energy and moreover it can be positive as well as negative. Here we explore t he role of impurities in the leads in determining the magnetoconductan ce of the loop. We find that the change in magnetoconductance is negat ive and can be made large provided the impurities do not create resona nt states in the systems. This theoretical finding may play a useful r ole in quantum switch operations.