CROSSOVER FROM ORTHOGONAL TO UNITARY SYMMETRY FOR BALLISTIC ELECTRON-TRANSPORT IN CHAOTIC MICROSTRUCTURES

We study the ensemble-averaged conductance as a function of applied ma gnetic field for ballistic electron transport across few-channel micro structures constructed in the shape of classically chaotic billiards. We analyse the results of recent experiments, which show suppression o f weak localization due to magnetic field, in the framework of random- matrix theory. By analysing a random-matrix Hamiltonian for the billia rd-lead system with the aid of Landauer's formula and Efetov's supersy mmetry technique, we derive a universal expression for the weak-locali zation contribution to the mean conductance that depends only on the n umber of channels and the magnetic flux. We consequently gain a theore tical understanding of the continuous crossover from orthogonal symmet ry to unitary arising from the violation of time-reversal invariance f or generic chaotic systems. (C) 1995 Academic Press, Inc.