Abstract
In ultrasmall devices where the relevant time scale is comparable to the dynamical time scale, the usual Markovian transport theory based on time coarse graining becomes untenable. We present a new non-Markovian quantum transport formalism that is not only valid in the dynamical regime, but also free from the energy runaway problem plaguing the Markovian theory. Apart from making practical calculations numerically stable, the theory provides the foundation for delineating the range of validity of the classical Boltzmann and Markovian quantum transport.