  Although devices working on quantum principles can revolutionize the
electronic industry, they have not been achieved yet as it is difficult to
control their stability. We show that one can use evanescent modes to build
stable quantum switches. The physical principles that make this possible is
explained in detail. Demonstrations are given using a multichannel Aharonov -
Bohm interferometer. We propose a new $S$ matrix for multichannel junctions to
solve the scattering problem.
