Transport channel of the electron beam in a short-wavelength free-electron laser is quite narrow and long, which is needed for the electrons to have excellent motion feature so as to prevent the transverse divergence. In this paper the characteristics of the ultra-relativistic electron motion in a planar wiggler filed with transverse distribution are studied by analytical derivation, nonlinear simulations and Kolmogorov entropy calculations. Results show that the wiggler not only forces the electron to periodically wiggle on the transverse plane, but also superpose a transverse shift motion leaving away from the wiggler axis, which leads to the transverse divergence of the electron motion in the absence of an external magnetic focusing system. However, it is shown that the proper choice of the initial value of the electron transverse velocity could suppress the transverse motion divergence, where the electron can smoothly pass through a narrow and long channel with a length of 10 m and a width of 0.09 mm and the electron motion is stable even if there is no external magnetic focusing system.