Evidence that Fowler-Nordheim field emission contributes significantly to the device current passing through electroformed metal-insulator-metal (MIM) devices at high voltage has been found by recording the low temperature current-voltage characteristics of Cu-SiOx-Cu devices, At low voltages (< 10 V) the conduction can be modelled by mechanisms in which, after emission from a given trap, the carriers are captured by the next trap in the filament chain (i.e., either 1-dimensional Poole-Frenkel conduction or tunnelling between adjacent traps). However, at higher voltages (10-15 V), we find evidence of an additional conduction mechanism, Fowler-Nordheim field emission. The onset of this mechanism coincides with the onset of electron emission, most of which occurs at near ballistic energies, indicating that a small fraction of this current is capable of traversing the remainder of the insulator and the top metal layer without significant energy loss.