  The breakdown of conductance quantization in a quantum point contact in the
presence of 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 much higher backscattering rate than any direct backscattering process.
For the realistic contact lengths ($\leq 2000$nm) the scattering processes
prove to be independent, in spite of coherence of the electron wave. The
distribution function of conductance fluctuations is obtained by direct
numerical calculations as well as 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 choice of parameters.
In particular, it is the better the larger the intermode distance is compared
to the amplitude of the random impurity potential.
