  We report and analyze low-temperature measurements of the conductance of
partially disordered reduced graphene oxide, finding that the data follow a
simple crossover scenario. At room temperature, conductance is dominated by
two-dimensional (2D) electric field-assisted, thermally-driven (Pollak-Riess)
variable-range hopping (VRH) through highly-disordered regions. However, at
lower temperatures T, we find a smooth crossover to follow the
exp(-E_0/E)^(1/3) field-driven (Shklovskii) 2D VRH conductance behaviour when
the electric field E exceeds a specific crossover value E_C (T)_2D = (E_a
E_0^(1/3) /3)^(3/4) determined by the scale factors E_0 and E_a for the
high-field and intermediate field regimes respectively. Our crossover scenario
also accounts well for experimental data reported by other authors for
three-dimensional disordered carbon networks, suggesting wide applicability.
