Breakdown of conductance quantization in quantum point contacts with realistic impurity potentials

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Abstract
The breakdown of conductance quantization in a quantum point contact in the presence of a random long-range impurity potential is discussed. It is shown that in the linear response regime a decisive role is played by the indirect backscattering mechanism via quasilocalized states at the Fermi level, this can provide a much higher backscattering rate than any direct backscattering process. For realistic contact lengths (<or=2000 nm) the scattering processes prove to be independent, in spite of the coherence of the electron wave. The distribution function of conductance fluctuations is obtained by direct numerical calculations as well as being estimated within an analytical model for the first time. It is shown to be a generalized Poisson distribution. Estimates are made for quantum point contact performance at different choices of parameters. In particular, it is better the larger the intermode distance is compared to the amplitude of the random impurity potential.