The electronic structure is determined for different carbon structures like carbon six chain, infinite graphene sheet, and different nanotubes. Electron transport properties of bare metallic carbon nanotubes (CNT) and a CNT with a single Barium atom are studied using the software Virtual NanoLab (VNL) and Atomistix ToolKit (ATK). The software combines advanced numerical methods, such as non-equilibrium Green's functions (NEGF) and density functional theory (DFT), to describe the detailed electronic structure of these carbon structures. Advanced numerical algorithms enable the application of a bias voltage across the structure, as well as the calculation and analysis of current-voltage characteristics of two-probe nanotubes.NanoLanguage scripts are written to generate and calculate the current for a two-probe bare CNT and a CNT with a single Barium atom. The currents thus obtained for different voltages are imported into Poly Software International (PSI) Plot spreadsheet and the current-voltage curves are plotted and compared for both the CNTs. The data is further analyzed and differential conductance is calculated and plotted for comparison. The current-voltage (IV) curves exhibit linearity for low voltages. Above a critical voltage, the IV curves flatten and exhibit features where sharp changes occur in the differential conductance. Adding a single Barium atom dramatically affects the conductance of the carbon nanotubes.