  We find the current voltage characteristics of a 2DEG-S interface in magnetic
field taking into account the surface roughness. Typically in experiments
$L/2R_c\gtrsim 3$, where $L$ is the surface length and $R_c$ is the cyclotron
radius. The conductance behaves in experiments usually as
$G=g_0+g_1\cos(2\pi\nu+\delta_1)$; higher harmonics,
$g_2\cos(4\pi\nu+\delta_2),...$, are hardly seen. Theories based on the
assumption of the interface perfectness can hardly describe qualitatively the
visibility of the condctance oscillations and the amplitudes of the harmonics:
they predict $g_1\sim g_2\sim g_3,...$. Our approach with the surface roughness
qualitatively agrees with experiments. It is shown how a disorder at a 2DEG-S
interface suppresses the conductance oscillations with $\nu$.
