Recent years have seen an increasing interest in end-to-end available bandwidth estimation. A number of estimation techniques and tools have been developed during the last few years. All of them can be classified into two models: rate-based and gap-based, according to the underlying approaches. The difference in characteristics of both models is studied in this paper. PathChirp and IGI are selected to represent each model and they are evaluated on low speed paths which are typical in today's local access of the Internet. Finally a hybrid method adopting both rate-based and gap-based approaches is proposed. The hybrid method is compared with pathChirp and IGI. Simulation proves that the hybrid method yields more accurate results and reduces overhead traffic.