A classification of half-metals is illustrated with examples of ferromagnetic and ferrimagnetic oxides. I : V characteristics and magnetoresistance of contacts between oxides with localized electrons (Fe₃O₄) and delocalized electrons ((La_0.7Sr_0.3)MnO₃) are discussed. The magnetoresistance is large when the conductance of the contact is less than the quantum of conductance G₀2e²/h. Broad distributions of point contacts are investigated in pressed powder compacts, illustrated by powder magnetoresistance measurements of CrO₂, (La_0.7Sr_0.3)MnO₃ and Fe₃O₄. When exchange coupling exists across an atomic scale contact, a narrow domain wall of width comparable to the contact size may be pinned there. Some consequences are thermal magnetic mode fluctuations, nonlinear I : V characteristics due to electron pressure on the wall and current-induced switching in a bistable `peanut' device with two closely spaced nanoconstrictions.