Abstract: The electronic transport experiments on topological insulators exhibit a dilemma. A negative cusp in magnetoconductivity is widely believed as a quantum transport signature of the topological surface states, which are immune from localization and exhibit the weak antilocalization. However, the measured conductivity drops logarithmically when lowering temperature, showing a typical feature of the weak localization as in ordinary disordered metals. Here, we present a conductivity formula for massless and massive Dirac fermions as a function of magnetic field and temperature, by taking into account the electron-electron interaction and quantum interference simultaneously. The formula reconciles the dilemma by explicitly clarifying that, the temperature dependence of the conductivity is dominated by the interaction, while the magnetoconductivity is mainly contributed by the quantum interference. The low temperature behaviors of conductivity in topological insulators indicate the localization of surface electrons in topological insulators. This demonstrates the breakdown of topological protection of surface states in an interacting topological insulator. The theory paves the road to quantitatively study the transport in topological insulators, and can also be extended to other two-dimensional Dirac-like systems, such as graphene, transition metal dichalcogenides, and silicene.