  As a result of sinusoidal spatial modulation, helimagnet induces
spin-dependent diffracted transmission. In this work, we propose a general
scattering matrix treatment to the transport properties in helimagnets with
arbitrary helical structures. Multiferroic properties can be considered by
taking electric polarization and magnetic helix together into account. A
monolayer magnetic helix can be treated as a $\delta$-barrier by diffraction
theory. The conductance and shot noise properties of the normal
metal/helimagnet/normal metal toy model are investigated. It is found that the
shot noise is suppressed and demonstrates rise-and-fall variations as a result
of interference between different diffracted channels. Sharp change of the shot
noise occurs when one and both diffracted beams disappear into evanescent modes
at the helimagnet spiral wave vector equal to one and two times the electron
Fermi wave vector $q=k_F$ and $2 k_F$. We also considered the conductance and
shot noise properties in a real multiferroic helimagnet $\rm{TbMnO_3}$ with
ferroelectric and helimagnetism coexistent. It is found that clockwise and
counterclockwise spin helix is distinctly separated in the conductance and shot
noise spectrum. Their variation pattern is a combined result of helimagnetism
and electric polarization.
