CONDUCTANCE FLUCTUATION THEORY FOR A MIXED TRANSPORT REGIME

We analyse the non-universal conductance fluctuations for a dirty quan tum wire in a transport regime where both mesoscopic and ballistic tra nsport characteristics play a role. This 'mixed' transport regime is r eached when impurities are distributed near the walls of a quantum wir e, leaving the centre region ballistic. Using a diagrammatic technique , we find that the existence of a ballistic region destroys the conven tional universal conductance fluctuations. The crossover behaviour of the fluctuation amplitude from the usual quasi-1D situation to that of the mixed regime is clearly revealed, and the role of various length scales identified. Our analytical predictions are confirmed by a direc t numerical evaluation of the Landauer-Buttiker formula.