  We investigate the role of local heating and forces on ions in the stability
of current-carrying aluminum wires. We find that heating increases with wire
length due to a red shift of the frequency spectrum. Nevertheless, the local
temperature of the wire is relatively low for a wide range of biases provided
good thermal contact exists between the wire and the bulk electrodes. On the
contrary, current-induced forces increase substantially as a function of bias
and reach bond-breaking values at about 1 V. These results suggest that local
heating promotes low-bias instabilities if dissipation into the bulk electrodes
is not efficient, while current-induced forces are mainly responsible for the
wire break-up at large biases. We compare these results to experimental
observations.
