  We have developed a fast, yet highly reproducible method to fabricate
metallic electrodes with nanometer separation using electromigration (EM). We
employ four-terminal instead of two-terminal devices in combination with an
analog feedback to maintain the voltage $U$ over the junction constant. After
the initialization phase ($U < 0.2V), during which the temperature $T$
increases by 80-150 degs C, EM sets in shrinking the wire locally. This quickly
leads to a transition from the diffusive to a quasi-ballistic regime ($0.2V < U
< 0.6V). At the end of this second regime, a gap forms (U > 0.6V). Remarkably,
controlled electromigration is still possible in the quasi-ballistic regime.
