The conductance of Si MOSFETs of submicrometer dimensions was studied as a function of source-drain voltage. Aperiodic structure was observed in the conductance as the gate voltage and magnetic field were varied. This structure was found to depend on the polarity of the source-drain voltage. The structure in the rectified portion (antisymmetric part) of the conductance was observed to have a magnetic field scale similar to that of the symmetric part. The magnitude of this structure (relative to the symmetric conductance structure) increased linearly at first, then sublinearly, with increasing source-drain voltage, as is predicted by the existing theories of conductance fluctuations due to quantum interference. © 1987.