\bibitem{Mair02}        R.W.~Mair, P.N.~Sen, M.D.~Hurlimann, S.~Patz, D.G.~Cory, R.L.~Walsworth,
                        {\it The Narrow Pulse Approximation and Long Length Scale Determination in Xenon Gas Diffusion NMR Studies of
                        Model Porous Media},
                        JMR {\bf 156}, 202, 2002.