The electrical observation of energy sub-band formation in the electronic structure, that gives rise to the phenomenon of quantized transport is reported in tin oxide (SnO2) nanobelt back-gate field-effect transistors, at low temperatures. Sub-band formation was observed as current oscillations in the drain current vs. gate voltage characteristics, and was analyzed considering the nanobelt as a "quantum wire" with a rectangular cross-section and hard walls. The lateral quantum confinement in the nanowires created conditions for the successive filling of the first twelve electron energy sub-bands, as the gate voltage increases. When the source-drain voltage is changed, the oscillations are not dislocated with respect to the gate voltage indicating flat-band energies, and that the observations are incompatible with the phenomena of Coulomb blockade and tunnelling oscillations. The separation of the energy sub-bands was found to be in good agreement with the measured cross-section dimensions of the nanobelt and with the threshold temperature, since for T > 60 K the oscillations tend to vanish.