Several possible physical origins of the magnetoresistance have been proposed.

1. A PRL earlier this year proposes is the material is a

**topological insulator**with gapless surface states described by a highly anisotropic Dirac cone.

2. In 1998 Abrikosov proposed the materials are gapless semiconductors with a linear spectrum, doped to a small carrier concentration, and that

**only one Landau level**contributes to the conductivity.

3. Parish and Littlewood's proposal that the key physics is that of a strongly

**spatially inhomogeneous semiconductor**which can be described by a random resistor network.

4. I also note that a band structure which produces a non-zero Berry curvature can produce a linear magnetoresistance, according to p. 1984 of this Rev. Mod. Phys. [However, I suspect this is a very small effect].

The challenge is to come up with experimental signatures which can distinguish between the four different theoretical proposals.

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