This work presents detailed characterization and modeling of the reset operation in resistive-switching memories based on metal oxides. Our experimental results confirm previous observations that reset is controlled by Joule heating, providing an insight on the electrical and thermal parameters of the conductive filament (CF) in the low resistance state. The characterization of such parameters allows to model the CF rupture responsible for reset switching. Our model explains the switching by self-accelerated dissolution of the CF, and can quantitatively account for reset and data-retention experiments. The scaling of programming current is finally investigated by means of reduction of CF cross-section.