The fabrication and gas flow characterization of an ultra-thin inorganic nanosieve structured by interference lithography and a bond-micromachining approach are reported. The nanosieve has been observed to exhibit transition gas flow behaviour around atmospheric pressure and ambient temperature. The small lip thickness (45 nm) of the nanopores with respect to their diameter (120 nm) helps in understanding pure transition flow by minimizing interactions between the molecule and inner pore wall. Due to the absence of these collisions, the transition flux is the superimposition of viscous and molecular fluxes without the need for higher-order slip correction. The nanosieve shows a flow selectivity of 3.1 between helium and argon at 20 mbar.