\bibitem{Codd99}        S.L.~Codd, P.T.~Callaghan,
                        {\it Spin Echo Analysis of Restricted Diffusion under Generalized Gradient Waveforms: 
                        Planar, Cylindrical and Spherical Pores with Wall Relaxivity},
                        J.~Magn.~Res. {\bf 137}, 358, 1999.