  A granular system slightly below the percolation threshold is a collection of
finite metallic clusters, characterized by wide spectrum of sizes, resistances,
and charging energies. Electrons hop from cluster to clusters via short
insulating "links" of high resistance. At low temperatures all clusters are
Coulomb blockaded and the dc-conductivity is exponentially suppressed. At
lowest T the leading transport mechanism is variable range cotunneling via
largest (critical) clusters, leading to the modified Efros-Shklovsky law. At
intermediate temperatures the principal suppression of the conductivity
originates from the Coulomb zero bias anomaly occurring, when electron tunnels
between adjacent large clusters with large resistances. Such clusters are
essentially extended objects and their internal dynamics should be taken into
account. In this regime the T-dependence of conductivity is stretched
exponential with a nontrivial index, expressed through the indices of
percolation theory. Due to the fractal structure of large clusters the anomaly
is strongly enhanced: it arises not only in low dimensions, but also in d=3
case.
