The Rammsauer-Townsend effect describes situations in which this occurs with unity probability.  
The Rammsauer-Townsend effect describes situations in which this occurs with unity probability.  The WKB approximation can be used to show that the probability of this occurring is exponentially dependent on twice the integral of the magnitude of the classical momentum over h-bar, although the classical momentum is imaginary in the relevant region.  This effect is the basis for a type of diode invented by Leo Esaki, while the Josephson effect depends on it occurring in an insulating oxide layer.  For a delta-function potential, its probability scales as the inverse of the quantity one plus the square of the wavelength of the incoming particle, while for finitely wide potentials, the probability of this happening drops off exponentially with the potential width.  First applied to alpha decay, FTP identify this quantum mechanical phenomenon in which particles may be spontaneously transmitted through a potential barrier.
The Rammsauer-Townsend effect describes situations in which this occurs with unity probability.  
quantum {tunneling}