The simplest model of quantum transport in devices is to describe the problem in terms of the scattering of the electron wavefunction by a spatially varying potential. One assumes that this potential is situated between two electron reservoirs, each of which emits particles with an equilibrium distribution into the scattering region. The reservoirs will, in general, have different chemical potentials, their difference representing an applied bias voltage. The net flux of electrons passing between the reservoirs constitutes the electrical current conducted by the device. A single-particle Schrödinger equation can only describe a situation in which the electrons move perfectly coherently throughout the device. Any loss of coherence due to inelastic collisions requires a higher-level description. Nevertheless, the solutions of Schrödinger 's equation remain one of the fundamental tools available to understand and predict the behavior of quantum-scale devices.