  We investigate whether the quantal and experimental amplitudes of current
oscillations of Resonant Tunneling Diodes in tilted fields are obtainable from
Periodic Orbit (PO) theories by considering recently proposed PO approaches. We
show, for the first time, that accurate amplitudes and frequency shifts for the
current oscillations (typically to within a few %) can be obtained from a
simple analytical formula both in the stable (torus-quantization) limit and the
unstable regimes of the experiments which are dominated by isolated PO's. But
we find that the PO approach does not describe quantitatively the dynamically
interesting intermediate experimental regimes which appear to be dominated by
contributions from complex orbits and multiple non-isolated PO's. We conclude
that these regimes will not easily be described by the usual PO approach, even
with simple normal forms.
