  Using a time operator, we define a tunneling time for a particle going
through a barrier. This tunneling time is the average of the phase time
introduced by other authors. In addition to the delay time caused by the
resonances over the barrier, the present tunneling time is also affected by the
branch point at the edge of the energy continuum. We find that when the
particle energy is near the branch point, the tunneling time becomes strongly
dependent on the width of the incoming wave packet, which implies that there is
no intrinsic tunneling time. This effect is related to the quantum uncertainty
in the particle's momentum.
