Summary: The scattering matrix approach is applied to the study of tunneling spectra in a metallic film in proximity with a superconductor. The abnormal minigap in tunneling conductance is attributed to the phase coherence of electrons and Andreev-reflected holes in the metallic film. The coherent transport in the metal leads to a fully opened minigap, whose width decreases with increase in the metallic film thickness, while the sequential component results in non-zero densities of states within the minigap. The calculated results provide a reasonable explanation for abnormal minigap behavior observed in recent experiments.