  Nanoscale electronic devices are of great interest for all kinds of
applications like switching, energy conversion and sensing. The objective of
this chapter, however, is not to discuss specific devices or applications.
Rather it is to convey the conceptual framework that has emerged over the last
twenty years, which is important not only because of the practical insights it
provides into the design of nanoscale devices, but also because of the
conceptual insights it affords regarding the meaning of resistance and the
essence of all non-equilibrium phenomena in general. We present a unified
description applicable to a wide variety of devices from molecular conductors
to carbon nanotubes to silicon transistors covering different transport regimes
from the ballistic to the diffusive limit, based on what we call the
NEGF-Landauer approach.
