We report on a theoretical study of quantum transport in carbon nanotubes in the presence of chemical-disorder-induced quasibound states, with a quantitative analysis of the relationship between the energy-dependent elastic mean free path and localization length. An external magnetic field applied perpendicularly to the nanotube axis is shown to induce a floating up in energy of the quasibound states, which results in giant magnetoconductance fluctuations under a Fermi level shift.