Hysteresis I-V is observed in our Cu-ZnO nanowire-Cu devices, the dynamic characteristics of which are measured across a very wide frequency band. The devices demonstrate a strong frequency dependent I-V. A working mechanism based on that of electromechanically tunable varistors is postulated by analyzing the experimental results. Electrostatic force induced by the external voltage was able to alter the location of the nanowires and in turn change the nonlinearity of the varistor. The theory also well elucidates the behavior of our devices driven at higher frequencies.