The conductance through an atomic-size point contact is examined using tight-binding models. In a case of single s band, we observe the conductance quantization in units of 2e2/h when the atomic structure changes gradually from leads to the point contact. In the presence of broad s and narrow s bands, the conductance still seems quantized. In the presence of s and p bands, however, the conductance is substantially suppressed by the interband scattering. This is because the hybridization between s and p orbitals in the electronic states around the point contact differs from that in the bulk Bloch states, which disturbs the adiabatic conduction through the point contact. Based on the calculated results, we discuss a possible mechanism for the quantized conductance in units of e2/h which has been observed experimentally with Ni break-junctions.
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