The lowest conductance step for a Ni nanocontact is anomalously small incomparison with the large expected number of conducting channels. We presentelectronic structure calculations for an extremely idealized Ni nanobridgeconsisting of just a monatomic nanowire. Our calculations show that no lessthan eight single spin bands cross the Fermi level in a nonmagnetic Nimonatomic wire, dropping marginally to seven in the more stable, fullyferromagnetic state. However, when we build in the wire a magnetizationreversal, or domain wall, by forcing the net magnetization to be zero, wesuddenly find that d electrons selectively cease to propagate across the wall.s electron propagation remains, and can account for the small observedconductance steps.