Here V=V0i,B=B0j Force on moving electron due to magnetic field is
F=−e(v×B)=−e=[V0i×B0J]−ev0B0k
As this force is perpendicular to v and B, so the magnitude of v will not change, i.e. momentum (=mv) will remain constant in magnitude.Hence, de-Broglie wavelength λ=h/mv remains constant