Using a three dimensional-quasi-one dimensional-three dimensional model to depict the point contact between a scanning tunneling microscope tip and a metal crystal, by indenting the tip into a metal surface and then withdrawing we have studied the electron transport properties of the microstructure. By means of the single electron approximation and the transfer matrix method, we solve the Schr?dinger equation for this three dimensional system, and calculate the variation regularity of its conductance by quantum mechanics method. The variation of the conductance with constricting and stretching of the constriction is studied, and the quantization of the conductance in units of 2e2/h and 2×2e2/h are obtained.