We fabricate ferromagnetic nanowires with constrictions whose cross section can be reduced gradually from 100 × 30 nm2 to the atomic scale and eventually to the tunneling regime by means of electromigration. The contacts are mechanically and thermally stable. We measure low-temperature magnetoresistances (MR) < 3% for contacts < 400 Ω, reproducible MR variations that are nonmonotonic in the regime 400 Ω − 25 kΩ, and a maximum MR of 80% for atomic-scale widths. These results for devices > 400 Ω differ from previous room-temperature studies of electrodeposited devices. For samples in the tunneling regime, we observe large fluctuations in MR, between −10 and 85%.