The (excess) power spectrum J(Gamma) of the depolarized component of scattered light was measured for 13 samples of atactic oligo- and poly(methyl methacylate)s (a-PMMA), each with the fraction of racemic diads f(r) = 0.79, in the range of weight-average molecular weight M(w) from 3.02 x 10(2) (trimer) to 7.55 x 10(3) in acetonitrile at 44.0 degrees C (Theta) and also for methyl isobutyrate in acetonitrile at 44.0 degrees C and in carbon tetrachloride at 25.0 degrees C. The spin-lattice relaxation time T-1 was also determined for the trimer through pentamer and the two samples with M(w) = 7.55 x 10(3) and 2.02 x 10(4), and the nuclear Overhauser enhancement NOE, for the trimer and the latter two samples, all in acetonitrile at 44 degrees C. It is found that, as in the case of atactic polystyrene (a-PS) previously studied, J(Gamma) may be well represented in terms of a single Lorentzian independently of M(w) and that the relaxation time tau(Gamma) as defined as the reciprocal of the half-width at half-maximum of J(Gamma) evaluated at infinite dilution increases with increasing M(w) and levels off to its asymptotic value in the limit of M(w) --> infinity, being consistent with the recent theoretical prediction on the basis of the helical wormlike (HW) chain model. A comparison is made of the present data for tau(Gamma), T-1, and NOE with the HW theory, and it is shown that the theory may explain satisfactorily the data in the range of M(w) over bar greater than or similar to 10(3), although semiquantitatively for tau(Gamma). For M(w) less than or similar to 10(3), the rigid sphere model having the radius equal to the apparent root-mean-square radius of gyration of the HW chain may give a good explanation of tau(Gamma) but not of T-1, indicating that the dynamic depolarized light scattering and nuclear magnetic relaxation cannot be described in terms of a common single relaxation time. However, there is shown to be an effective (mean) magnetic relaxation time tau(M) which is approximately equal to 0.6 tau(Gamma). From a comparison of the present results for tau(Gamma) for a-PMMA with the previous ones for a-PS, the dynamic chain stiffness, as defined as the ratio of the relaxation time associated with the local motion of a long polymer chain to that of its isolated repeat unit (monomer), is found to be larger for a-PMMA than for a-PS, and it is shown that this difference may be well explained by the HW theory.