  The original proof has a gap, and need extra hypothesis that the strong
stable and strong unstable filiation both to be $C^1$. The argument is like the
following: with the regularity, one can show that the weak-stable and
weak-unstable foliation both to be $C^{1+Lip}$, and then following the same
argument as in the paper one can conclude the proof. But this extra hypothesis
seems implying that the flow has a contact structure. Then it will be only a
result which improves the Foulon's proof on contract structure for $C^\infty$
regularity to $C^2$, not so interest.
