Electron transport in thin (3.85.8-nm) silicon dioxide films has been investigated with use of the carrier-separation and vacuum-emission techniques. For the thinner films and smaller voltages, ballistic transport is inferred not only indirectly from the oscillations of the current-voltage characteristics (as originally reported by Lewicki and Maserjian) but also directly from the average energy measured via the carrier-separation technique and from the observation of the ballistic peak (about 0.15 eV wide) in the vacuum-emission experiments. In thicker films and/or at higher bias voltages, the transition from the ballistic to the steady-state regime is observed. Phonon replicas for the 0.153- and 0.063-eV longitudinal optical phonons have been observed in this transient situation. The experimental results have been compared to Monte Carlo simulations. Best agreement has been obtained by using a value of 0.7m0 for the polaron mass. The average mean free path of about 1 nm resulting from the simulation is in good agreement with Lewicki and Maserjians experimental data. © 1987 The American Physical Society.