Effective description of hopping transport in granular metals
  We develop a theory of a variable range hopping transport in granular conductors based on the sequential electron tunnelling through many grains in the presence of the strong Coulomb interaction. The processes of quantum tunnelling of real electrons are represented as trajectories (world lines) of charged classical particles in d+1 dimensions. We apply the developed technique to investigate the hopping conductivity of granular systems in the regime of small tunneling conductances between the grains g << 1. 
I.S. Beloborodov, A.V. Lopatin, V.M. Vinokur, V.I. Kozub