  The traditional transmission coefficient present in the original Landauer
formulation, which is valid for quasi-static scenarios with working frequencies
below the inverse of the electron transit time, is substituted by a novel
time-dependent displacement current coefficient valid for frequencies above
this limit. Our model captures in a simple way the displacement current
component of the total current, which at frequencies larger than the inverse of
the electron transit time can be more relevant than the particle component. The
proposed model is applied to compute the response of a resonant tunneling diode
from 10$\,$GHz up to 5$\,$THz. We show that tunneling electron devices are
intrinsically nonlinear at such high frequencies, even under small-signal
conditions, due to memory effects related to the displacement current. We show
that these intrinsic nonlinearities (anharmonicities) represent an advantage,
rather than a drawback, as they open the path for tunneling devices in many THz
applications, and avoid further device downscaling.
