The electronic structure at the surface of Bi(111) enables us to study the effect of defects scattering into multiple channels. By performing scanning tunneling spectroscopy near step edges, we analyze the resulting oscillations in the local density of electronic states (LDOS) as function of position. At a given energy, forward and backward scattering not only occur simultaneously but may contribute to the same scattering vector Δk. If the scattering phase of both processes differs by π and the amplitudes are almost equal, the oscillations cancel out. A sharp dip in the magnitude of the Fourier transform of the LDOS marks the crossover between forward and backward scattering channels.