  Nontrivial band topologies in semimetals lead to robust surface states that
can contribute dominantly to the total conduction. This may result in reduced
resistivity with decreasing feature size contrary to conventional metals, which
may highly impact the semiconductor industry. Here we study the resistivity
scaling of a representative topological semimetal CoSi using realistic band
structures and Green's function methods. We show that there exists a critical
thickness d_c dividing different scaling trends. Above d_c, when the defect
density is low such that surface conduction dominates, resistivity reduces with
decreasing thickness; when the defect density is high such that bulk conduction
dominates, resistivity increases in as conventional metals. Below d_c, the
persistent remnants of the surface states give rise to decreasing resistivity
down to the ultrathin limit, unlike in topological insulators. The observed
CoSi scaling can apply to broad classes of topological semimetals, providing
guidelines for materials screening and engineering. Our study shows that
topological semimetals bear the potential of overcoming the resistivity scaling
challenges in back-end-of-line interconnect applications.
