  We present a first-principles investigation to show that the contact
conductance of a half conductance quantum ($G_0/2$) found previously does not
generally hold for single-atom magnetic junctions composed of a tip and an
adatom dsorbed on a surface. The contact conductance of the Ni-Co/Co(111)
junction is approximately $G_0/2$, while for the Co-Co/Co(111), Ni-Ni/Ni(111),
and Ni-Ni/Ni(001) junctions the contact conductances are 0.80$G_0$, 1.55$G_0$,
and 1.77$G_0$, respectively. The deviation from $G_0/2$ is mainly caused by the
variation of the spin-down conductance largely determined by the minority $d$
orbitals, as the spin-up one changes little for different junctions.
