What is a factor of 2 to theorists?
There is an interesting preprint The normal state of URu$_2$Si$_2$: spectroscopic evidence for an anomalous Fermi liquid by Tom Timusk and collaborators.
From a measurement of the frequency dependency of the conductivity they aim to extract the frequency and temperature dependence of the scattering rate of the Fermi liquid quasi-particles. General considerations suggest it has the form:
The authors find b ~ 1 but suggest that Landau would have b=4.
A few comments:
1. A fuller discussion of the theoretical literature is in a review Quantum criticality in organic conductors? Fermi liquid versus non-Fermi-liquid behaviour by Martin Dressel.
The Kondo Problem cites work on the Anderson single impurity model which gives b=1 [see equation 5.102].
3. The Kadowaki Woods ratio [as discussed here] will be proportional to b.
4. I believe that measurements of the frequency and temperature dependence of ultrasound (zero sound) attenuation in liquid 3He are consistent with b=4.
I thank Nigel Hussey (who is currently visiting UQ) for bringing the paper to my attention.