Electronic conduction in an atomic sheet is examined theoretically from first principles as an example of a typical two-dimensional system. The conductance per atomic chain, calculated assuming a bias voltage of 0.1 V, is 0.87 G₀ for the atomic sheet, which is lower than for the corresponding single chain (1.0 G₀; G₀ = 2e²/h). The difference is attributed to a reduction of the local density of states around the Fermi level, particularly in the region where the bridge comes into contact with the electrode. [DOI: 10.1380/ejssnt.2006.507]