SOFT DISORDER EFFECTS IN THE CONDUCTANCE QUANTIZATION IN QUANTUM POINT CONTACTS - INDIRECT BACKSCATTERING STATISTICS

The breakdown of conductance quantization in a quantum point contact i n the presence of random long-range impurity potential is discussed. I t is shown that in the linear response regime a decisive role is playe d by the indirect backscattering mechanism via quasilocalized states a t the Fermi level; this can provide a much higher backscattering rate than any direct backscattering process. For realistic contact lengths (less than or equal to 2000 nm) the scattering processes prove to be i ndependent, in spite of coherence of the electron wave. The distributi on function of conductance fluctuations is obtained by direct numerica l calculations as well as estimated within an analytical model. It is shown to be a generalized Poisson distribution. The quantum point cont act performance is estimated for different choices of parameters. In p articular, the larger the intermode distance in comparison with the am plitude of the random impurity potential, the better is the performanc e.