We investigate electron scattering by acoustic phonons in rectangular quantum wires, paying special attention to the coherent interference between the deformation potential scattering and the ripple mechanism scattering due to wire-surface vibration. We show that the deformation potential and the ripple mechanism scatterings do not take place independently but always occur coherently, taking account of the normal phonon modes of the wire. The ripple mechanism raises the scattering rates in particular for thin wires composed of material having a small electron effective mass. We show the significance of the ripple mechanism in electron scattering in quantum wires, by illustrating the electron scattering rates of square and rectangular GaAs and InAs wires.