Physical mechanism of superluminal traversal time: interference between   multiple finite wave packets
  The mechanism of superluminal traversal time through a potential well or potential barrier is investigated from the viewpoint of interference between multiple finite wave packets, due to the multiple reflections inside the well or barrier. In the case of potential-well traveling that is classically allowed, each of the successively transmitted constituents is delayed by a subluminal time. When the thickness of the well is much smaller in comparision with a characteristic length of the incident wave packet, the reshaped wave packet in transmission maintains the profile of the incident wave packet. In the case of potential-barrier tunneling that is classically forbidden, though each of the successively transmitted constituents is delayed by a time that is independent of the barrier thickness, the interference between multiple transmitted constituents explains the barrier-thickness dependence of the traversal time for thin barriers and its barrier-thickness independence for thick barriers. This manifests the nature of Hartman effect. 
Xi Chen and Chun-Fang Li