Modular recursive Green's function method for ballistic quantum transport

A modification of the standard recursive Green's function method for quantu m transport through microstructures is presented which is based on the deco mposition into separable substructures. The Green's functions for these mod ules are joined by discretized Dyson equations. Nonseparable structures can thereby be calculated with the help of a few recursions with high accuracy . We apply this method to the calculation of ballistic quantum transport th rough a circular and stadium-shaped quantum dot for high mode numbers to te st semi-classical predictions in detail. Among other results we find the br eakdown of the semiclassical approximation for long path lengths which is d ue to the spreading of wave packets in the cavity.