  We report on pulsed and DC I-V characteristics of polycrystalline samples of
three charge-ordered manganites, Pr_{2/3}Ca_{1/3}MnO_3, Pr_{1/2}Ca_{1/2}MnO_3,
Bi_{1/2}Sr_{1/2}MnO_3 and of a double-perovskite Sr_2MnReO_6, in a temperature
range where their ohmic resistivity obeys the Efros-Shklovskii variable range
hopping relation. For all samples, the DC I(V) exhibits at high currents
negative differential resistance and hysteresis, which mask a perfectly ohmic
or a moderately nonohmic conductivity obtained by pulsed measurements. This
demonstrates that the widely used DC I-V measurements are usually misleading.

Strongly Correlated Electrons