We consider semiconductor devices that are composed of two parts: first, a small quantum structure constituting the active region, and second, a classical environment with larger typical length scales, While in general the classical environment should be represented by a drift-diffusion model, we consider here only simple contacts which we take as ideal metals with infinite conductivity, The transport through the quantum structure is described as in the Landauer-Buttiker formation through electronic scattering wave functions which define the electron density in the quantum system. Further sources of the self-consistent Coulomb field are layers of classical charges in the contacts at each of the interfaces to the quantum system. We present further a capacitance model that takes into account the openness of the quantum structure and the existence of finite contacts embedding the system. As particular quantum structures we study simple tunneling barriers.