  The tunneling time of particle through given barrier is commonly defined in
terms of "internal clocks" which effectively measure the interaction time with
internal degrees of freedom of the barrier. It is known that this definition of
the time scale for tunneling is not unique in the sense that it depends on the
clock used to define it. For the case of resonance tunneling, a particular
choice that in the limit of a high/broad square barrier yields the original
result of Buttiker and Landauer (Phys. Rev. Lett. 1982, 49, 1739) is correlated
to the lifetime of the resonance state. This is illustrated for analytically
solvable one-dimensional double barrier models and for a realistic model of
electron tunneling through a static water barrier. The latter calculation
constitutes a novel application of this concept to a 3-dimensional model, and
the observed structure in the energy dependence of the computed traversal time
reflects the existence of transient tunneling resonances associated with
instantaneous water structures. These models, characterized by the existence of
shape resonances in the barrier, make it possible to examine different internal
clocks that were proposed for measuring tunneling times in situations where a
"clock independent" intrinsic time scale (the resonance life time) for the
tunneling time exists. It is argued that this time may be used in order to
estimate the relative importance of dynamical barrier processes that affect the
tunneling probability.
