Quantum-mechanical wavepacket transport in quantum cascade laser   structures
  We present a viewpoint of the transport process in quantum cascade laser structures in which spatial transport of charge through the structure is a property of coherent quantum-mechanical wavefunctions. In contrast, scattering processes redistribute particles in energy and momentum but do not directly cause spatial motion of charge. 
S.-C. Lee, F. Banit, M. Woerner, and A. Wacker