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We present radio observations of a comprehensive sample of 90 dwarf stars andbrown dwarfs ranging from spectral type M5 to T8. We detect three radio activesources in addition to the six objects previously detected in quiescence andoutburst, leading to an overall detection rate of about 10% for objects laterthan M7. From the properties of the radio emission we infer magnetic fieldstrengths of ~100 G in quiescence and nearly 1 kG during flares, while themajority of the non-detected objects have B<50 G. Depending on theconfiguration and size of the magnetic loops, the surface magnetic fields mayapproach 1 kG even in quiescence, at most a factor of few smaller than inearly-M dwarfs. With the larger sample of sources we find continued evidencefor (i) a sharp transition around spectral type M7 from a ratio of radio toX-ray luminosity of log(L_R/L_X) ~ -15.5 to >-12, (ii) increased radio activitywith later spectral type, in contrast to H-alpha and X-ray observations, and(iii) an overall drop in the fraction of active sources from about 30% for Mdwarfs to about 5% for L dwarfs, fully consistent with H-alpha and X-rayobservations. Taken together, these trends suggest that some late-M and Ldwarfs are capable of generating 0.1-1 kG magnetic fields, but the overall dropin the fraction of such objects is likely accompanied by a change in thestructure of the chromospheres and coronae, possibly due to the increasinglyneutral atmospheres and/or a transition to a turbulent dynamo. A more extendedradio survey currently holds the best promise for measuring the magnetic fieldproperties of a large number of dwarf stars. [abridged]Comment: Submitted to ApJ; 14 pages, 4 figures, 2 table

Radio Observations of a Large Sample of Late M, L, and T Dwarfs: The Distribution of Magnetic Field Strengths