Given the wave function
$\psi(z, t) = Ae^{i(kz - \omega t)} + Be^{-i(kz + \omega t)}$
where A and B are constants, demonstrate that the probability current density is
$\mathbf{J} = v(|A|^2 - |B|^2)\hat{z}$
where $v = \frac{\hbar k}{m}$. Interpret the result physically.