QUANTUM EFFECTS AND THE ROLE OF THE POTENTIAL RIDGEIN IONIZATION THRESHOLD LAW Naoyuki Miyashita$^{*\dagger}$, Daiji Kato$^\sharp$ , Shinichi Watanabe$^\dagger$, Michio Matsuzawa$^\dagger$, and J.~H.~Macek$^\ddagger$ $^\dagger$Department of Applied Physics and Chemistry, U. of Electro-Communications, 1-5-1 ~Chofugaoka, Chofu-shi, Tokyo 182-8585, Japan $^\sharp$Japan Science and Technology Corporation, International Project ``COLD TRAPPED IONS'' AxisChofu Bldg 3F, 1-40-2, Fuda, Chofu-shi, Tokyo 182-0024, Japan $^\ddagger$Department of Physics and Astronomy, University of Tennessee,Knoxville, Tennessee 37996, U.S.A. and Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, U.S.A. Do quantum effects distinct from the classical counterpart show up in electron-atom ionization near the threshold?We investigate the threshold behavior of two model electron-atom ionization problems, one in which the target atom interacts with the incidentelectron only by monopole (Temkin-Poet model) and the other in which the nuclear charge is 1/4. These models help us identify purely quantum mechanical effects as well as the limitation of the semi-quantal treatment based on the expansion of the Hamiltonian near the ridge.