SIZE-EFFECTS AND TUNING OF PHASE COHERENCE IN A DOUBLE-QUANTUM-WELL MESOSCOPIC WIRE

We have investigated the magnetoresistance properties of mesoscopic wi res fabricated from a modulation-doped GaAs/(AlGa)As double quantum we ll structure containing two coupled 2D electron gases (2DEGs). An in-p lane magnetic held, B, ''tunes'' the tunnelling between the 2DEGs and, in a Hall bar device, leads to a resistance feature at B approximate to 10T due to a van Hove singularity in the density of states. In meso scopic wires this feature becomes a strong resistance peak. We attribu te this enhancement to a size effect. In addition, we observe universa l conductance fluctuations with the in-plane B. These disappear when B is strong enough to suppress the tunnelling between the wells. (C) 19 98 Elsevier Science B.V. All rights reserved.