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Distorted-wave calculations of the triply differential cross sections for electron-impact ionization of H, He, Ne, Ar, Kr, and Xe are presented for final-state electrons sharing ≤4 eV excess energy and for the coplanar, θ12=π geometry. Results for H and He targets, presented briefly elsewhere [C. Pan and A. F. Starace, Phys. Rev. Lett. 67, 185 (1991)], are analyzed in greater detail and compared with available experimental and other theoretical results. In particular, for He targets the effects of exchange and of second-order capture processes are exhibited. These are shown to be of lesser importance than other short-range effects on the s-wave phase shifts of both incident and final-state continuum electrons for reproducing the experimentally observed target dependence of the electron angular distributions for H and He targets [P. Schlemmer, T. Rösel, K. Jung, and H. Ehrhardt, Phys. Rev. Lett. 63, 252 (1989)]. More approximate theoretical (e,2e) results for Ne, Ar, Kr, and Xe targets also show striking target dependences; they are compared with available experimental results.