  Effects of impurity scatterings on the conductance in normal-metal / $d$ wave
superconductor junctions are discussed by using the single-site approximation.
So far, the split of the zero-bias conductance peak has been believed to be an
evidence of the broken time reversal symmetry states at the surface of
high-$T_c$ superconductors. In this paper, however, it is shown that the
impurity scattering near the interface also causes the split of the zero-bias
conductance peak. Typical conductance spectra observed in experiments at finite
temperatures and under external magnetic fields are explained well by the
present theory.
