  We propose a first-principles method of efficiently evaluating
electron-transport properties of very long systems. Implementing the recursive
Green's function method and the shifted conjugate gradient method in the
transport simulator based on real-space finite-difference formalism, we can
suppress the increase in the computational cost, which is generally
proportional to the cube of the system length to a linear order. This enables
us to perform the transport calculations of double-walled carbon
nanotubes~(DWCNTs) with 196,608 atoms. We find that the conductance spectra
exhibit different properties depending on the periodicity of doped impurities
in DWCNTs and they differ from the properties for systems with less than 1,000
atoms.
