We study the hopping conductivity in a composite made of Gaussian coils of flexible metallic wires randomly and isotropically suspended in an insulator at such concentrations that the spheres containing each wire overlap. Uncontrolled donors and acceptors in the insulator lead to random charging of wires and, hence, to a finite density of states at the Fermi level. Then the Coulomb interactions between electrons of distant wires result in the soft Coulomb gap. At low temperatures the conductivity is due to variable range hopping of electrons between wires and obeys the Efros-Shklovskii (ES) law $\ln \sigma \propto -(T_{\mathrm{ES}}∕T{)}^{1∕2}$ with $T_{\mathrm{ES}}$ decreasing with concentration and length of the wires. Due to enhanced screening of Coulomb potentials, at large enough wire concentrations and sufficiently high temperatures, the ES law is replaced by the Mott law $\ln \sigma \propto -(T_M∕T{)}^{1∕4}$.

• Received 18 July 2006

DOI:https://doi.org/10.1103/PhysRevB.74.174201