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We have studied negative-resistance mixers both theoretically and experimentally. The current carried by the bound quasi-particles in the N-region of a mesoscopic SNS weak-link has a dc and cosine components. Current transport in the SNS weak-links is affected by bias-voltages in many experiments. The I-V curves often show complex structures including negative differential resistance which cannot be understood by the simple RSJ model. Supercurrent is directly affected both by the bias voltage and the density of states at the energy gap of superconductor. The supercurrent by the Multiple-Andreev-Reflection is considerably changed across subharmonic gap voltages because of the singularity of the density of state of superconductor near the energy gap. We have done mixer experiment of SNS weak-links and have observed IF peaks at subgap voltages which are caused by the negative differential resistance.