1. The problem statement, all variables and given/known data When an electron has tunnelled through a potential barrier it's wavefunction is described by a plane wave travelling in the positive x direction. In this region the probability density is constant. I am trying to explain why it is constant but can't find any info in books or on the internet. 2. Relevant equations f(x)=Fexp(ikx) 3. The attempt at a solution Have had some ideas which haven't worked out to be right. I now it is something to do with the wavefunction being a plane wave but can't work out why it is constant.