\bibitem{Hagslatt03}    H.~Hagsl\"att, B.~J\"onsson, M.~Nyd\'en, O.~S\"oderman,
                        {\it Predictions of Pulsed Field Gradient NMR Echo-Decays for Molecules Diffusing in Various Restrictive Geometries.
                        Simulations of Diffusion Based on a Finite Element Method},
                        JMR {\bf 161}, 138, 2003.