More About Tunnelling Times, the Dwell Time, and the ``Hartman Effect"

In a recent review paper [{\em Phys. Reports} {\bf 214} (1992) 339] we proposed, within conventional quantum mechanics, new definitions for the sub-barrier tunnelling and reflection times. \ Aims of the present paper are: \ (i) presenting and analysing the results of various numerical calculations (based on our equations) on the penetration and return times $<\tau_{\, \rm Pen}>$, $<\tau_{\, \rm Ret}>$, during tunnelling {\em inside} a rectangular potential barrier, for various penetration depths $x_{\rm f}$; \ (ii) putting forth and discussing suitable definitions, besides of the mean values, also of the {\em variances} (or dispersions) ${\rm D} \, {\tau_{\rm T}}$ and ${\rm D} \, {\tau_{\, \rm R}}$ for the time durations of transmission and reflection processes; \ (iii) mentioning, moreover, that our definition $<\tau_{\rm T}>$ for the average transmission time results to constitute an {\em improvement} of the ordinary dwell--time ${\ove \tau}^{\rm Dw}$ formula: \ (iv) commenting, at last, on the basis of our {\em new} numerical results, upon some recent criticism by C.R.Leavens. \ \ We stress that our numerical evaluations {\em confirm} that our approach implied, and implies, the existence of the {\em Hartman effect}: an effect that in these days (due to the theoretical connections between tunnelling and evanescent--wave propagation) is receiving ---at Cologne, Berkeley, Florence and Vienna--- indirect, but quite interesting, experimental verifications. \ Eventually, we briefly analyze some other definitions of tunnelling times.