  In the present paper, we propose a mean field approach for spin ladders based
upon the Jordan-Wigner transformation along an elaborately ordered path.
  We show on the mean field level that ladders with even number legs open a
energy gap in their low energy excitation with a magnitude close to the
corresponding experimental values, whereas the low energy excitation of the
odd-number-leg ladders are gapless. It supports the validity of our approach.
We then calculate the gap size and the excitation spectra of 2-leg-ladder
system. Our result is in good agreement with both the experimental data and the
numerical results.
