In a Young's double slit experiment, an electron beam is used to obtain interference pattern. If the speed of electron decreases, then:

Fringe width, $\mathrm{\beta }=\frac{\mathrm{\lambda D}}{\mathrm{d}};$ Also, $\mathrm{\lambda }=\frac{\mathrm{h}}{\mathrm{mv}}$

Here $\mathrm{h}$ is Planck's constant. This wavelength is inversely proportional to the velocity. As the speed decreases, wavelength increases. Hence, the fringe width increases with decrease in electron speed.