A non-quantum interference mechanism in the current-voltage characteristics of double-barrier semiconductor heterostructures is suggested. It is based on the admixing of independent contributions of channels corresponding to different values of the electron momentum along the plane of device. The disturbing of coherence of channels due to non-parabolicity of the conduction band and magnetic field parallel to the plane causes a modulation of amplitude of resonant oscillations (looking like beats) in devices with wide interbarrier space. In the simplest cases the amplitude of oscillations depends on the voltage drop V and magnetic field H as sin(V3/2)/V3/2 and J1(H)/H respectively. Our consideration gives an interpretation of the experiments in a parallel magnetic field.