Physics Review D
When the full conformal algebra is gauged there arises a gauge field for inverse translations in addition to the usual translational gauge field. By considering every scale-invariant action constructible from the curvatures of the conformal group and the metric, we show that when the gauge field of the usual translations is identified as the vierbein, the gauge field of inverse translations may always be eliminated by its own field equation. After this elimination, every torsion-free, scale-invariant action reduces to a linear combination of the square of the conformal curvature tensor Cαβμν and the square of the Weyl field strength Cαβ.
Wheeler, J. T., Auxiliary field in conformal gauge theory, Phys Rev D44 (1991) 1769 - 1773. http://prola.aps.org/abstract/PRD/v44/i6/p1769_1