  Statistics of closed paths in two-dimensional systems, which just determines
the interference quantum correction to conductivity and anomalous
magnetoconductance, has been studied by computer simulation of a particle
motion over the plane with randomly distributed scatters. Both ballistic and
diffusion regimes have been considered. The results of simulation have been
analyzed in the framework of diffusion approximation. They are used for
calculation of the magnetic field dependence of magnetoconductance in the model
2D system. It is shown that the anomalous magnetoconductance can be in
principle described by the well known expression, obtained in the diffusion
approximation, but with the prefactor less than unity and phase breaking which
differs from true value.
