It is well known that a current proportional to $J\propto\sin(\phi_1-\phi_2)$ flows when two superconductor with phases $\phi_1$ and $\phi_2$ are connected. Also, a current which depends on the phases according to $\sin[(\phi_1-\phi_2)/2]$ is claimed to be fractional Josephson effect (which is believed to be a signature of Majorana fermions). But I see that even with two s-wave superconductors connected through a barrier, the current could be proportional to $\sin[(\phi_1-\phi_2)/2]$ (see for instance - http://arxiv.org/abs/cond-mat/9811026) when the barrier is zero. I do not understand this well. Does it mean that $J\propto\sin(\phi_1-\phi_2)$ is true only in the large barrier limit ? Then, what is holy about fractional Josephson current?